Examples of organic carbonates are cyclic alkylene carbonates (such as ethylene carbonate) and non-cyclic dialkyl carbonates (such as diethyl carbonate). It is well known to make cyclic alkylene carbonate by reaction of alkylene oxide (such as ethylene oxide) with carbon dioxide in the presence of a suitable catalyst. Such processes have been described for example in U.S. Pat. No. 4,508,927 and U.S. Pat. No. 5,508,442.
Dialkyl carbonates can be produced by reaction of alkylene carbonate with alkanol. Where alkylene carbonate (such as ethylene carbonate) is reacted with alkanol (such as ethanol), the products are dialkyl carbonate (such as diethyl carbonate) and alkanediol (such as monoethylene glycol). Such process is well-known and an example thereof is disclosed in U.S. Pat. No. 5,359,118. This document discloses a process in which di(C1-C4 alkyl) carbonates and alkanediols are prepared by transesterification of an alkylene carbonate with a C1-C4 alkanol.
An example of an alkanol impurity that may be contained in an organic carbonate stream is an ether alkanol, for example an alkoxy alkanol. JP2003300917 and JP2002371037 relate to processes wherein dimethyl carbonate and monoethylene glycol are made from ethylene carbonate and methanol and wherein 2-methoxyethanol is formed as a by-product. In the inventions of JP2003300917 and JP2002371037, said 2-methoxyethanol is removed by specific distillation techniques.
At various points within said total process producing dialkyl carbonate from alkylene oxide via alkylene carbonate, organic carbonate streams containing one or more alkanol impurities may be produced. An example of such alkanol impurity is an ether alkanol, for example an alkoxy alkanol. For example, in a reactor where ethanol and ethylene carbonate are reacted into diethyl carbonate and monoethylene glycol, a side-reaction of ethanol with ethylene oxide, formed by back-reaction of ethylene carbonate into ethylene oxide and carbon dioxide, into 2-ethoxyethanol (ethyl oxitol) may take place. Further, ethyl oxitol may be formed by a side-reaction of ethanol with ethylene carbonate in such a way that carbon dioxide is released and ethyl oxitol is produced. Still further, a side-reaction between ethanol and monoethylene glycol may take place producing ethyl oxitol and water. Still even further, ethyl oxitol may be formed via decarboxylation of hydroxyethyl ethyl carbonate.
Therefore, the product stream from a reactor where ethanol and ethylene carbonate are reacted into diethyl carbonate and monoethylene glycol, may comprise unconverted ethanol, unconverted ethylene carbonate, diethyl carbonate, monoethylene glycol and the above-mentioned ethyl oxitol impurity. The presence of said alkoxy alkanol impurity may be detrimental in any subsequent production process. Said alkoxy alkanol impurity may for example end up in the dialkyl carbonate that is used as a starting material for the synthesis of diphenyl carbonate from said dialkyl carbonate and phenol. For example, in a case where the dialkyl carbonate is diethyl carbonate and the alkoxy alkanol impurity is ethyl oxitol, said ethyl oxitol may react with the phenol starting material and/or with the diphenyl carbonate product.
Direct reaction of phenol and ethyl oxitol may result in the production of phenyl 2-ethoxyethyl ether, and hence in the loss of valuable phenol reactant. Further, such reaction results in the introduction of undesired chemicals in the process and therefore to separation issues.
Reaction of diphenyl carbonate with ethyl oxitol results in product loss as phenyl 2-ethoxyethyl carbonate is produced. Further, the latter product acts as a “poison” in any subsequent polymerisation of diphenyl carbonate into polycarbonate material. For example, when diphenyl carbonate is reacted with bis-phenol A (BPA), polycarbonate and phenol are formed. Diphenyl carbonate can react with BPA since phenol is a relatively good leaving group. Dialkyl carbonates (such as diethyl carbonate) however cannot be used to produce polycarbonate by reaction with BPA, since alkanols are not good leaving groups. Alkoxy alkanols (such as ethyl oxitol) are neither good leaving groups. Therefore, in case phenyl 2-ethoxyethyl carbonate is present in a diphenyl carbonate feed to be reacted with BPA, phenol will be released easily from said phenyl 2-ethoxyethyl carbonate but not ethyl oxitol which will consequently stop the polymerization process at one end of the chain. Consequently, phenyl 2-ethoxyethyl carbonate has to be removed from diphenyl carbonate before the latter is contacted with BPA.
The above exemplifies that in a case where an organic carbonate stream containing an alkanol impurity is formed, it is desired to remove said alkanol impurity before any subsequent process takes place wherein the organic carbonate is transformed into a valuable end product. For example, it is needed to remove any ethyl oxitol impurity from a diethyl carbonate stream containing said impurity before reaction of the diethyl carbonate with phenol takes place.
Referring to the above example where ethanol and ethylene carbonate have been reacted into diethyl carbonate and monoethylene glycol, the product stream also containing unconverted ethanol and ethylene carbonate and ethyl oxitol side-product, may be separated by means of distillation. The boiling points for the various components in said product stream are mentioned in the table below.
ComponentBoiling point (° C.)ethanol78.4diethyl carbonate126-128ethyl oxitol135monoethylene glycol197.3ethylene carbonate260.4
The distillation as referred to above may result in a top stream containing diethyl carbonate and unconverted ethanol and a bottom stream containing monoethylene glycol and unconverted ethylene carbonate. Most likely, all of the ethyl oxitol ends up in the top stream. However, depending on the specific conditions under which distillation is carried out, part of the ethyl oxitol may end up in the bottom stream. Subsequently, said top stream may be further separated by means of distillation into a top stream containing unconverted ethanol which can be recycled to the reactor where diethyl carbonate and monoethylene glycol are produced, and a bottom stream containing diethyl carbonate and the ethyl oxitol impurity.
As discussed above, before an organic carbonate is transformed into a valuable end product in any subsequent process, the alkanol impurity has to be removed therefrom as that might interfere with said subsequent process and/or any further processes. For the above example, this means that the ethyl oxitol impurity should be removed from the bottom stream containing diethyl carbonate and the ethyl oxitol impurity. In principle, ethyl oxitol and diethyl carbonate could be separated by means of a further distillation step. However because of the small difference in boiling point between diethyl carbonate and ethyl oxitol (see above table), such separation is very cumbersome requiring many distillation steps and stages. Therefore, there is a need to find a simple method of removing an alkanol impurity from an organic carbonate stream containing such alkanol impurity.